Postfix alias entire domain (*.newdomain.tld -> *.olddomain.tld)

I have a customer who has around 100 mailboxs + all the aliases. He has now purchased a new domain name and requires all email address in the new domain to be sent to the old domain which will remain the master. Alias *.newdomain.tld -> *.olddomain.tld.
At the moment the only way I can see to do this would be to create an alias for each mailbox and aliases in the old domain

I found this on the internet, where would be the safest place to put it in the Ispconfig 3 files. is it safe to just edit the /etc/postfix/virtual file.?
"You can do this by inserting a @domain.from: @domain.to line into your virtual aliased map, which is in most cases in /etc/postfix/virtual. After that, you shouldn't forget to rerun a postmap virtual command to recreate its database!"